Natural arm dynamics play a critical role during ambulation, specifically for ensuring efficient and stable walking and avoiding a fall when faced with sudden perturbation. Accordingly, restriction of arm swing during able-bodied gait has been observed to increase metabolic cost, impair lateral stability, and interrupt natural walking dynamics. Importantly, arresting a fall wit outstretched arms is a common strategy for avoiding bodily injury, with falls causing 97% of upper limb fractures in older adults and 76% of wrist fractures resulting from direct impact to the hand. Consequently, the importance of natural and responsive arm dynamics for efficient and safe ambulation may have significant implications for persons with upper limb loss when not using a prosthesis or only wearing a prosthesis for specific daily activities. Whereas the vast majority of research on upper limb prostheses users has focused on functional use (e.g., reaching and grasping), little attention has been paid to their gait and standing balance characteristics. Consequently, the impact of upper limb loss on walking and standing, particularly in relation to the ability to avoid a fall, remains poorly understood. In light of the importance of arms in maintaining upright balance, we hypothesize that individuals with upper limb loss will exhibit altered ambulatory characteristics compared to able-bodied persons that will create instability and increase their risk of falls and fall-related injuries. Anecdotal evidece from clinicians has supported this hypothesis. Therefore, the primary objective of this project is to explore the impact of upper limb loss on the risk of falls and characteristics of gait and standing balance. To achieve this objective, qualitative and biomechanical data will be collected on individuals with upper limb loss. Qualitative data on etiology, mobility and balance confidence, and fall history will be collected with a questionnaire on individuals with unilateral r bilateral upper limb loss at or proximal to the level of the wrist. The questionnaire will be administered via the internet, telephone interviews, and hardcopy format. Biomechanical data of steady-state gait and standing balance will be collected on upper limb prosthesis users with unilateral limb loss above and below the elbow while: 1) using their current prosthesis, 2) not using a prosthesis, and 3) using an experimental mock (i.e., nonfunctional) prosthesis that matches the inertial properties of the sound limb. Observed biomechanical data will include those previously shown to be associated with fall risk and instability in elderly and other pathological populations prone to unsteadiness. Within-group differences will be analyzed through appropriate repeated measures statistical methods and regression analysis will be applied to develop a predictive model for falls in individuals with upper limb loss. As there is a dearth of information available on the impact of upper limb loss on falls and characteristics of walking and standing, results from this project will provide much-needed insight into this topic and quantify an undocumented health hazard to this population. Subsequent studies will then focus on identifying the underlying mechanisms of instability in upper limb prosthesis users. The long term goal is to develop effective rehabilitation strategies and enhanced technology (e.g., improved prosthesis designs) that facilitate efficient and safe movement and lead to reduced falls, increased daily participation, and an improved quality of life.